This invention relates to improved magnetic recording media of the type that are known as discs and is particularly advantageous when embodied in those flexible disc products known as floppy discs as well as polymodal.
In reading the following discussion of the background of the invention, it must be remembered that the discussion has been prepared with a full knowledge of the invention. It cannot be, and is not intended to be, a view of the existing and as apparent to one of ordinary skill in the art at a time preceding the invention.
The magnetic recording members, subject of this invention, comprise a coating of magnetic particles in a matrix such as an organic polymer matrix. There are two general kinds of such members. A first kind is typified by the most common type of magnetic tape. This kind of product moves in a longitudinal direction in relationship to a recording or reading head because information is recorded sequentially along the length of the medium. The other kind of product is a disc which rotates while the reading head is positioned radially or helically, much in the relationship of a needle on a phonograph disc. These products are very well recognized in the art and have each been in extensive commercial use for some time. There has been a great deal of effort expended over the last thirty years or so to improve the magnetic recording characteristics of such products. One such procedure has been the orientation of the magnetic particles within their matrix. A discussion of such orientation is presented in The Physics of Magnetic Recording by C. D. Mee; 1968; North-Holland Publishing Company, Amsterdam. Both longitudinal and vertical orientation is discussed. Longitudinal orientation is orientation along the direction of travel of the recording member, e.g. along a tape. Vertical orientation would be the same particles "standing up" on end, i.e. normal to the tape surface. Mee points out that use of orienting techniques is not indicated for applications such as magnetic discs "which require tape to be magnetized in different directions." Among additional publications disclosing various orientation techniques and procedures are U.S. Patents 3,052,567 to Gabor et al; 3,185,755; 2,796,359; 3,256,112; 3,117,065; 2,711,901; 3,261,706; 3,065,105 and 3,627,680. U.S. Pat. Nos. 3,117,065; 3,052,567 and 3,185,775 discuss vertical orientation of magnetic particles. There is somewhat related art, typified by U.S. Pat. No. 3,001,891, wherein an AC orienting field is used to allow a "free" orientation of particles in response to other fields.
In some known art, discs have included oriented particles. This is especially true of inadvertent radial orientation of rigid (as opposed to floppy) spin-coated magnetic discs. Also, U.S. Pat. Nos. 3,256,112 and 3,001,891 disclose the use of an orienting procedure and state that discs may be oriented. In such a procedure, the particles have always been oriented in a circular direction, i.e. in a direction directly analogous to the longitudinal direction in tapes.
The flexible articles known as floppy discs are formed on a coating apparatus exactly like magnetic tape, then cut into circular discs. Such discs carry unoriented particles and cannot take advantage of any high magnetic squareness attributes of said particles. One example of such a disc format is described in "IBM Diskette, Original Equipment Manufacturers' Information" Product Reference Literature Number GA21-9190-1, File GENL-19 available from IBM Corporation.
In still another area of the magnetic recording art, there has been a great deal of attention paid to the development of superior magnetic particles. Usually such particles are cobalt-based metal particles like those dislosed in U.S. Pat. Nos. 3,909,240 to Deffeyes et al; 3,574,685 to Haines et al; and 3,607,218 to Akashi et al. Such particles can be utilized in providing much superior magnetic recording members than are possible using the traditional iron oxide or "chromium dioxide" particles. However, it is difficult to form magnetic recording members which fully exploit the potential of the improved metallic particles on commercial mixing coating equipment operated at acceptable output rates.
A major problem, in this respect, is the adequate dispersal of such powders. A number of inventors have provided improved means for dispersing such particles more efficiently. For example, U.S. Pat. No. 3,026,215 suggests the lengthwise and breadthwise orientation of such particles. Manly, in U.S. Pat. No. 3,172,776 discloses a process wherein chains of metal particles are formed to facilitate their being adapted more readily to an orienting procedure. A somewhat similar suggestion is made in U.S. Pat. No. 3,228,882 to Harle et al. Akashi et al, in U.S. Pat. No. 3,740,266 suggests still another method for overcoming problems inherent in orienting the more acicular of said particles.
While the problem of coating defects has slowed down the commercial application of such metal powders in magnetic tape, especially magnetic tapes of the polymodal type, it has been a particular barrier to using the powders in floppy discs wherein a defect in the coating will necessarily cause a large area of processed tape to be discarded.
In the disclosure set forth below, the inventors will describe in improved magnetic disc and process for making the same. The novel disc product embodies attributes substantially overcoming the limitations of the disc products described above. Appicants, in solving the problem associated with such discs, have discovered a general solution to the roping problems heretofore associated with both acicular particles and small metallic particles, a solution useful in both disc and tape products. Also of interest in the prior art are magnetic powder combinations used as polymodal materials and disclosed in copending and commonly owned U.S. Patent Application, Ser. No. 411,253 filed Oct. 31, 1973 by Deffeyes.
That application discloses a system based on the use of a magnetic identification medium formed of at least two distinct populations of ferromagnetic powders, wherein a first population is selected so that it can be magnetically switched, i.e. recorded upon, by a magnetic field at which a second population will not be switched. Each population is also responsive to differing stimuli, say, differing magnetic fields or temperatures, for erasing information therefrom; that is, magnetic information can be retained by one population as magnetic information is erased from another population. Such systems are referred to herein as polymodal systems. It is not intended to include within the term "polymodal" those systems which have a supplemental magnetic coating only for the purpose of enhancing a signal as opposed to bearing a separate signal.
In general, it is desirable to have populations in a bimodal system characterized by coercivity values which have a difference of at least 200. In practice, bimodal systems may be selected to have differences of 1000 or greater. If additional functionality is built into a ferromagnetic system by adding different modes, the higher and lower coercivities will usually differ by at least a factor of 200 (X-1), wherein X is the number of modes, and it will be advantageous to keep the coercive force difference between the different populations at 200 oersteds or more.
The discovery that certain combinations of powders can be utilized in achieving such distinctly polymodal ferromagnetic systems was unexpected in view of the performance of mixtures formed of the ferromagnetic powders most generally used in the art. For example, a mixture formed of a first iron oxide, a second iron oxide and cobalt-based metal powders having coercivities of 180, 320, and 1000, respectively, yielded a low coercivity peak which was not distinct although there were distinct peaks between the low coercivity materials and the high coercivity material.
The above result may be partly understood in view of the teaching that the peaks should be about 200 oersteds apart. However, the two low coercivity powders interact more than would be expected on the sole basis of the insufficient difference in coercivities.
In general, it appears that the metal powders, i.e. those of Bm values exceeding about 8000 gauss, are the most advantageous for use in forming bimodal systems, in that they can be used successfully in combination with oxides and other metal powders.
Some typical Bm values are 3400 for gamma Fe.sub.2 O.sub.3, 4000 for black Fe.sub.3 O.sub.4, about 16,000 for the cobalt metal powder of Example 1 below and about 13,000 for the low coercive force metal powder of Examples 2 and 3.
In general, the peaks visible on a non-integrated dM/dt curve of an advantageous polymodal system can usually be recognized in two ways:
1. As one raises the field on a BH meter the peak of the lower coercivity curves will be substantially complete before the appearance of the next higher curve.
2. The valleys between adjacent peaks are advantageously of a depth equal to at least one-half of the average height of the adjacent peaks over a base line.
Previously it has been known, or at least alleged to have been known, to make multi-layer bimodal magnetic tapes using thin films of plated metal. Such recording media, disclosed in U.S. Pat. Nos. 3,328,195 and 3,219,353 to May and Prentky, give little or no evidence that any high performance bimodality was achieved. In any event, manufacture of the May and Prentky products require relatively expensive, multi-layer construction and also utilize metallic films, the formulation and design of which are not only very complex and expensive, but depend for their magnetization upon different considerations, e.g. the controlled movement of domain walls, than do magnetic particles dispersed in a resin matrix. Moreover, such films are not readily varied as to composition and are not believed as susceptible to enhancement of recording properties by use of bias currents because eddy currents seem to be induced, possibly because of the relatively high conductivity of metal films.
Other deficiencies in the bimodal materials of Prentky and May are the necessity of using very large coercivity differences. Trimodal or quatramodal media become economically and technically impractical using their processes and product.
Moreover, it appears that Prentky and May had no idea that materials, or at least those exhibiting substantial shape anisotropy could be used in polymodal materials. The Prentky and May materials exhibit little or no shape anisotropy although they exhibit a great deal of magnetic interaction because of the nature of domain-wall-modulated magnetic recording.
It is further noted that in an article entitled "The Effect of Particle Interaction on the Coercive Force of Ferromagnetic Micropowders" (Proceedings of the Royal Society, A, 232 (1189), pages 208-226, 1955), Wohlfarth discloses that "the change of coercive force with packing is negligible for powders controlled by magnetocrystalline anisotropy . . . but is significant for particles controlled mainly by shape anisotropy." This statement is related to the present invention only by a hindsight search for a theory by which to further explain some of the unexpected and advantageous attributes of the products of the invention.
In further hindsight review of art bearing some relationship to fields relating in some superficial structural or utilitarian aspect to the invention, the following art was located:
U.S. Pat. No. 3,601,913 to Pollock suggests a device utilizing a mixture of high and low coercivity materials. The utility of this system depends on the detection of magnetic voids caused by mechanical displacement of magnetic powder bearing surfaces. Pollock also suggests that use of such mixtures will make card counterfeiting more difficult. But Pollock uses his mixture only to complicate the manipulative acts of duplicating a card; the individual components serve no distinct functions except in instances wherein they are at least partly non-congruent so that they yield a visually-identifiable pattern or a geometrically distinct identifying pattern formed of one of the magnetic powders. In general, the powders simultaneously resond to the field as a single-population powder would.
Moreover, the chromium oxide and iron oxide mixture of the Pollock patent would be inoperable in forming combinations for use in such processes as are to be described below.
U.S. Pat. No. 3,790,754 to Black teaches use of adjacent ferromagnetic coatings to achieve a multi-modal, but relatively insecure magnetic recording system. The system is generally similar to Pollock's system.
In recently-issued U.S. Pat. No. 3,761,311 to Perrington et al, there is described a dual layer magnetic tape, each layer having different nominal coercivities. It is believed that such a tape is sold under the trade designation Scotch Brand C-60 Cassette Cobalt-Energized High-Energy type (Catalog Number S-C-60ME) by 3M Company. That tape is not bimodal, probably because magnetic interactiion substantially pervades the ultra-thin layer of the tape. Moreover, since it was developed for, and is sold to, a specific audio recording market, it is clearly not intended for, nor tolerant of any substantial bimodal character.
In the following description, the term "card" will be used in the same sense of a mechanical equivalent of any article which carries magnetic material in information-yielding arrangement. It will be obvious that the scope of the kinds of members which can be tagged with magnetic identification means is very broad and that such recording members are mechanical equivalents to the credit card referred to in this application.
Also, U.S. Pat. Nos. 3,986,205 and 3,986,206 to Fayling, based on applications filed subsequently to the parent application of Applicant, describe a special mode of bimodality whereby particular magnetic particles such as barium ferrite are utilized in a particular way to form bimodal materials. Barium ferrite is a peculiar material exhibiting a very high coercivity in one direction in comparison to the coercivity exerted in a direction normal thereto. The coercivity ratio is about 2.5:1 whereas the ratio in most magnetic materials is below 1.5:1 and usually about 1.25:1. This large difference in coercivity allows Fayling, using special processing procedures, to achieve a polymodal recording system having at least some polymodal characteristics.
Fayling utilizes his barium ferrite and like materials as one component of a polymodal system. Fayling's polymodal recording media cannot be processed in the usual way, i.e. using a single ring type erase-head of the type normally used in the magnetic recording art. This is true with respect to tape processing. Moreover, a spatially large erase field must be used in processing a credit card-sized object by the Fayling system.
Finally, Fayling's system does not appear to be useful on magnetic discs without the development of wholly new processing procedures.
It should also be noted that both operable powder populations of Fayling are believed to exhibit shape anisotropy. This is a consequence, it is believed, of chip-like barium ferrite flakes (normally not having shape anisotropy) aligning themselves into a "stack" formed of a plurality of the chips when they are mixed and oriented in a polymer matrix.
U.S. Pat. No. 3,566,356 to Holm et al discloses some magnetic recording media using two magnetic materials in the same composition. However, the purpose of these inventions is to provide a positive interaction between the two populations to assure a characteristic and distinctive coercivity characteristic. Holm et al also appear to disclose some two-layer particulate magnetic materials which seem to be capable of some bimodal performance. This Holm et al bimodality requires a difference in coercivity but the characteristic is achieved by cross-orienting two layers of iron oxide.